A non-linear junction is a junction between different materials for which a change in the voltage applied across the junction does not produce a proportional change in the current flowing through the junction. These non-linear junctions are found in semiconductor components such as diodes, transistors and integrated circuits. However, non-linear junctions also occur naturally and can be detected in rock, building material crystals, and metal/oxide junctions. One common source of non-linear junctions is the large number of structural fasteners such as nails, screws, bolts and the like that are almost always found in buildings. These non-linear junctions distort electrical signals that pass through the junction. Subjecting such a non-linear junction to a strong high frequency radio signal causes an electric current to flow through the non-linear junction. The non-linearity in the junction causes a distortion of the originating radio signal thereby generating signals at harmonic frequencies of the incoming signal. At least a second harmonic signal at twice the transmitted frequency and a third harmonic signal at three times the transmitted frequency are typically re-radiated when a non-linear junction is subjected to a high frequency radio signal.
A non-linear junction detector is a device that is used in a fashion similar to a metal detector. However, while a metal detector is designed to detect metal, a non-linear junction detector is designed to detect semiconductor non-linear junctions in electronic devices such as diodes and transistors. A non-linear junction detector works on the principle that by radiating a non-linear junction with sufficiently powerful signals, detectable signals at integral multiples of the frequency of the signal originally radiated on the non-linear junction will be produced and re-radiated by the non-linear junction. Because they occur at harmonic frequencies of the original radiated signal, these re-radiated signals are known as harmonics or harmonic signals. By detecting the presence of these harmonics, the non-linear junction detector can detect the presence of a non-linear junction.
One application that has been devised for non-linear junction detectors is to affix a tag containing a non-linear junction to items in a store that are likely to be shoplifted. A non-linear junction detector is then placed at the exit of the store. When a customer purchases an item, the tag containing the non-linear junction is removed. If the non-linear junction tag is not removed before the item is taken from the store, an alarm will sound when the tag passes through the non-linear junction detector to inform the store's employees that an item is being stolen. Thus, non-linear junction detection technology has been adapted for shoplifting prevention.
Non-linear junction detectors have also been used to detect and locate covert surveillance devices that may be hidden in a room for the purpose of making audio or visual recordings of the activities occurring in the room. These covert surveillance devices contain electronic devices that have non-linear junctions. To locate the surveillance devices, the non-linear junction detector is waved in a sweeping pattern over all the areas in which a surveillance device may be hidden. When the device is waved above a non-linear junction, an alarm notifies the user that a non-linear junction has been detected. One major advantage to using a non-linear junction detector for counter surveillance measures is that it is possible to detect a hidden bugging device even if the device is not turned on and operating.
However, these non-linear junction detectors suffer from several drawbacks. For example, non-linear junction detectors typically utilize linearly polarized antennas. Using linearly polarized antennas forces the user to scan surfaces in both a horizontal and vertical motion to ensure that a surveillance device is not present with a polarized harmonic return that is perpendicular to the non-linear junction detector's receiver function. The additional time required to scan an area in both a horizontal and vertical fashion results in additional time and money being expended to search an area for surveillance devices.
Current models of non-linear junction detectors generally have a single antenna head that contains at least two antennas. One antenna is for transmitting the original signal and the other is for receiving a re-radiated harmonic of the transmitted signal. If the non-linear junction detector has the capability to receive both the second and third harmonics of the re-radiated signal, then three antennas are utilized, one antenna to receive the second harmonic, one to receive the third harmonic, and one to transmit the original frequency signal. Multiple cables are used to connect these multiple antennas to the transmitter and receivers of the non-linear junction detector. In addition, more cables are needed to support a display on the antenna head that informs the user when a non-linear junction has been detected. These additional cables tend to interfere with the use of the non-linear junction detector. Furthermore, multiple cables and antennas lead to increased cost and complexity in the non-linear junction detector.
Finally, prior art non-linear junction detectors tend to produce an excessive number of false alarms due to the presence of dissimilar metal junctions. This results in an increased amount of time and money needed to search a given area for surveillance devices. Furthermore, because dissimilar metal junctions may occur practically anywhere, the unknown type of non-linear junction may be inside of a wall or under a floor where it is very difficult to determine the type of non-linear junction present without causing damage to the surroundings. As used herein, "dissimilar metals" refers to corroded metal such as a rusty nail or any other material made up of two touching dissimilar metals, such as galvanized steel.